Traditionally, three approaches have been employed for connecting integrated circuits (ICs) to printed circuit boards. These approaches are wire bonding, chip carriers with beam leads, and direct chip connections. Flip chip technology is one of the direct chip connection approaches. In general, a flip chip assembly forms a direct electrical connection between an electronic component and a substrate, circuit board, or carrier, by means of conductive bumps on chip bond pads of the electronic component.
Flip chip technology has been used since the early 1960's. Compared to other packaging methods, flip chip technology provides increased performance in several areas, such as smaller package sizes, increased performance in high frequency applications, improved thermal capabilities, more I/O connections on a smaller die, and increased reliability. With these advantages, flip chip technology has found applications in the computer industry, automotive applications, electronic watches, cellular phones, pagers, and the like.
There is a constant push in the industry to achieve smaller die packages. Unfortunately, traditional flip chip assembly techniques have characteristics that limit the minimum thickness that can be achieved. One problem relates to the common use of the relatively thick, rigid polymer substrates. With the current technologies, by bonding of the silicon ICs to a rigid polymer substrate, devices currently in commercial production are on the order of 1 millimeter (mm) in thickness.
Flexible polymer substrates have been used in some approaches. However, previous direct chip connection approaches using thin flexible polymer substrates have typically been unsuccessful. One problem relates to uneven surfaces common to flexible polymer substrates. Such flexible substrates do not have the proper flatness for accurately bonding ICs using surface mount technologies (e.g., flip chip) with traditional approaches to die assembly. Because the polymers are so thin, it is difficult to manufacture polymers having sufficiently flat surfaces.
Hence, there exists a need in the art for apparatus and methods for flattening the thin flexible substrates to facilitate accurate bonding of ICs with the substrate.